Steps to reproduce:
1. Go to http://beta.musicbrainz.org/release/add?release-group=de208292-8db5-3aed-a14a-b37a84d8c521
2. Use the first release duplicate
3. Go to Add Disc -> Existing tracklist -> Search -> click next a couple times, select "Please, Please, Please (12" Vinyl 1) by James Brown and The Famous Flames" and add the disc
4. Open track parser for the (just added) disc 2, copy tracklist
5. Paste copied tracklist into track parser for disc 1, parse tracks
The track "Please Please Me" disappears and there's an unrecoverable error because the (invisible) track can't be edited.